1. Field of the Invention
The present invention relates to optical transmitting components, and more particularly to an apparatus for varying the light transmittance of optical components.
2. Description of the State of the Art
It is known to use an electrochromic (EC) device or liquid nematic crystals (LC) to control the transmittance of light through optical transmitting components. These materials have been widely applied to building and automobile windows largely due to the existence of convenient external power supplies which are necessary to change the translucence of the EC or LC material.
Portable optical transmitting devices with variable light absorption require self-contained power sources or materials that react directly to ambient light levels so that movement is not restricted by connection to a stationary power supply. Examples of such an optical transmitting device that utilize variable absorptance and non-restricted movement can be found in the eyewear field. Presently, the most common lens, which allows for both variable light absorption and mobility, is constructed with photochromic glass (also called phototropic or identified under the trademark PHOTOGRAY.TM.). A disadvantage of photochromic glass is that it can not be used in conjunction with plastic lenses. Photochromic glass includes a light-sensitive chemical structure which chemically darkens upon exposure to ambient light, but regains its original transparency over a period of time after the light is removed. Photochromic glasses, however, have the further disadvantage of a relatively slow response time. Full darkening requires approximately 5 minutes, while approximately 60 seconds is necessary to darken to 75% of the maximum density. A greater disadvantage is that recovery to the initial transparency is an even slower process. To achieve a 50% recovery, five minutes is required, while a 75% recovery requires twenty minutes.
Such slow response time required by photochromic glass is inadequate for use in eyewear, because the sight of an individual may be severely hampered when entering a dark area from an area of high intensity lighting. Also, since light in the ultraviolet (UV) spectrum causes the chemical reaction that varies light absorption in photochromic devices, the reaction is severely hampered by other glazing that blocks UV light, such as automobile windshields, energy efficient windows, and the like, that have been coated with UV absorbing coatings.
There have been some apparati and methods developed for controlling the transmittance of light through optical transmitting devices which operate with self-contained power sources. For example, U.S. Pat. No. 4,279,474 to Belgorod discloses a spectacle lens comprising a liquid crystal layer and electrode assembly sandwiched between a pair of glass lenses. A battery supplying a voltage across the electrodes is responsible for varying the optical density of the lens. The Russell et al. patent, U.S. Pat. No. 4,968,127 discloses eyewear comprising a pair of lenses having a liquid crystal layer therebetween, a power supply, and a control circuit having a phototransistor for sensing ambient light. The power source utilized by Russell et al., is a photocell array distributed about the periphery of the eyeglass frame. Okaue et al., in his U.S. Pat. No. 5,015,086 discloses electronic sunglasses having variable transmittance employing liquid crystal material and a solar cell as a power source.
The Ballmer patent, U.S. Pat. No. 4,531,814 discloses the use of an electrochromic material coating applied to optical transmitting components in combination with a battery and electric control circuit for increasing and decreasing the absorptance of the component.
Belgorod, Russell and Okaue teach the use of an optical transmitting device with variable absorbance having a self-contained power supply in combination with liquid nematic crystals. A disadvantage of liquid nematic crystal technology is that a constant source of energy is required to change the translucence of the lens, thus requiring a substantial and continuous source of electrical power as long as translucence instead of transparency is desired. The photocell arrays utilized by Russell and Okaue also have a further disadvantage of not being aesthetically pleasing or practical for all types of eyeware frame designs.
While Ballmer teaches the use of electrochromic coatings for controlling the transmittance of light through optical components his device, to be operational, requires an external battery for a current supply to be attached to the optical component.